Damping between two parts or bodies is often performed by a fluid filled ram or damping unit having a cylindrical bore or main body attached to one of the relatively moving parts and a rod attached to the other relatively moving part. The rod is attached to a piston running inside the bore and forming on one side a compression chamber which reduces in volume as the rod retracts and the damping unit compresses, and on the other side a rebound chamber which reduces in volume as the damping unit extends. The piston typically includes valving to control flow of fluid between the compression and rebound chambers. A common example is the telescopic “shock absorber” used between each wheel of a vehicle and its body to damp motions of the body and oscillations of the wheels. The fluid used as a damping medium can be a gas or a liquid such as hydraulic oil. There are two common constructions of hydraulic shock absorber, the mono tube which has a gas reservoir separated by a piston from the fluid in the compression chamber, or the twin tube in which the gas reservoir is located in a sleeve around the bore or piston cylinder and communicated with the compression chamber by a reservoir damper valve.
As the damping unit compresses, fluid flows from the compression chamber into the rebound chamber through the piston compression valving which uses holes and flexible shims to provide fluid restriction causing an increase in the fluid pressure in the compression chamber compared to the rebound chamber. These pressures acting over the piston generate a force opposing the compression motion, ie a compression damping force. Similarly, as the damping unit extends, fluid flows from the rebound chamber into the compression chamber through the piston rebound valving which uses a different set of holes and flexible shims to generate a rebound damping force.
In hydraulic shock absorbers the damping forces are generally dependent on the velocity of the rod motion relative to the piston cylinder, whereas with a gas filled shock absorbers, the damping forces are also frequency dependent.
In conventional shock absorber designs, the area of piston face used to accommodate the piston compression valving is similar or even the same as the area of piston face used to accommodate the piston rebound valving. However the compression and rebound damping forces required are rarely the same, with rebound commonly three times higher than compression damping force to limit input of large forces into the vehicle body structure. Also the effective area of the piston face in the compression chamber is larger than the effective area of the piston face in the rebound chamber. As a result, the area of holes providing a significant element of the damping force are usually much smaller in the rebound valving than in the compression valving.
This fundamental imbalance in piston valving area required in compression compared to rebound is addressed in the applicant's earlier U.S. Pat. No. 7,513,490 in which the compression and rebound flows are radially separated. Compression flow passes through an outer ring of holes and past large diameter flexible shims into the rebound chamber. Rebound flow passes from the rebound chamber through radial holes into a passage inside the rod of the damping unit and on into a central chamber inside the piston, from which one or more rebound damping holes (inside the outer ring of compression damping holes) permit flow into the compression chamber past small diameter flexible shims. This design inherently provides a larger flow area and lower damping force in compression than in rebound motions, as is typically required.
All of the above described shock absorbers are pressurised in use, and as the rod only extends through the rebound chamber giving a large effective piston face area on the compression chamber side, this pressure acts on the rod area to provide a force tending to extend the rod out of the damping unit (also known as a “push-out” force).
In the applicant's prior US patent, the use of a passage inside the rod typically increases the minimum rod diameter, which in turn increases the push out force. Although some push-out force is typically present as noted above, the primary function of the shock absorber is to provide damping. Increasing the push out force can alter the characteristics of the unit for example in light vehicles and/or with temperature changes and can make the units difficult to install in vehicles.
It would therefore be desirable to provide a damping unit having radially separated compression and rebound piston flows in which the minimum rod diameter can be reduced.
It would also be desirable to provide an improved construction of gas reservoir for a mono-tube hydraulic damping unit.